Donald Trump’s feud with top diplomat Rex Tillerson burst back into the open Tuesday, with the US president suggesting he and his Secretary of State compare IQ scores.

Having loudly dismissed reports that Tillerson once called him a “moron,” Trump showed no sign of letting the controversy go, renewing questions about Tillerson’s future as America’s top diplomat.

Just to make it clear that he’s smarter than his secretary of state, Trump suggested taking a test to prove it.

“I think it’s fake news,” Trump told Forbes magazine of Tillerson’s reported insult. “But if he did that, I guess we’ll have to compare IQ tests. And I can tell you who is going to win.”

The explosive interview was published hours before the two men were scheduled to meet at the White House for lunch with Secretary of Defense James Mattis.

Ahead of that sit-down Trump insisted he still had confidence in the secretary of state, saying “I did not undercut anybody. I don’t believe in undercutting people.”

But White House insiders said that Tillerson’s refusal to directly deny an NBC News report that he labelled Trump a “moron” after a July meeting at the Pentagon, only fueled differences between the men.

Since then White House chief of staff John Kelly has been struggling to keep a lid on the crisis — an effort that has been consistently thwarted by Trump’s tweets and barbed remarks.

After the reports, Trump took to Twitter to publicly upbraid the former ExxonMobil CEO for “wasting his time” trying to negotiate with North Korea.

The Twitter rebuke revived rumours that Tillerson is unhappy at his post, but he insists he has no intention of resigning.

– Diplomatically crucial time –

In Washington, Tillerson, along with Mattis, Kelly and chairman of the joint chiefs Joseph Dunford are increasingly seen as a buffer around Trump that contains an impulsive president.

Kelly has worked to control the flow of information that crosses Trump’s desk and imposed a decision-making structure that was absent in the early days of the administration.

“The White House has become an adult day care centre,” Senator Bob Corker declared at the weekend, in an astonishing public rebuke from a Republican who campaigned for Trump and chairs the Senate foreign relations committee.

Tillerson’s departure would be a major blow to those hoping to temper Trump and stop what Corker described as “the path to World War III.”

And it could not come at a more sensitive time diplomatically. Trump is poised to confront Iran by questioning a major nuclear deal later this week and appears set on upping tensions with North Korea.

Tillerson is also set to play a major role in preparing Trump’s monster trip to Asia next month, that will take in Japan, South Korea, China, Vietnam and the Philippines.

Still, it remains far from clear how long a secretary of state who has lost the ear of the president can remain in the post.

“When Cabinet officials continue to work for a president with whom they have fundamental disagreements, nothing good ever really comes of it,” Julian Zelizer, a history and public affairs professor at Princeton University wrote this week.